As monomers for ophthalmic use, the following silicone compounds are known.

The afore-mentioned TRIS, 3-[tris(trimethylsiloxy)silyl]propyl methacrylate, has poor compatibility with hydrophilic monomers, such as, 2-hydroxyethyl methacrylate (HEMA). Therefore, when TRIS is copolymerized with hydrophilic monomers, such a drawback occurs that a transparent polymer is not obtained. Meanwhile, SIGMA described above has good compatibility with hydrophilic monomers such as HEMA. Their copolymers are characterized by a relatively high oxygen transmission rate and a hydrophilic property. However, recently, a higher oxygen transmission rate is required for a polymer for ophthalmic use so that a user can continuously wear contact lenses for a longer term. The polymers obtained from SiGMA are insufficient in terms of the oxygen transmission rate.
In order to solve this problem, the compound represented by the following formula (a), hereinafter referred to as “(a)”, was proposed in the following Patent Literature 1.

The weight ratio of the Si-containing moiety, i.e., bis(trimethylsiloxy)methylsilyl in SIGMA and tris(trimethylsiloxy)silyl in (a), to the whole molecule is 52% for SIGMA and 60% for (a). The higher weight ratio of the Si-containing moiety seems to give the higher oxygen transmission rate.
SiGMA is a trimeric silicone which has three silicon atoms and (a) is a tetrameric silicone which has four silicon atoms. It seems that tetrameric or higher one is preferred for oxygen transmission. The weight ratio of the Si moiety in TRIS, tetrameric silicone, is 70% with a purity of 98% is commercially available.
When the weight ratio of the Si moiety is increased in order to increase the oxygen transmission rate, the molecular weight per polymerizable group increases and, therefore, strength of the copolymer decreases. Tetrameric and pentameric silicones seem to be preferable in order to attain both good oxygen transmission rate and strength of the copolymer.
The afore-mentioned (a) is prepared by reacting a corresponding epoxy precursor with methacrylic acid. There is a problem that many side reactions occur, so that properties of the resulting copolymers are various.
Meanwhile, as a method for the preparation of a silicone which is vinyl-polymerizable, a method is known from Patent Literature 3 where a cyclic siloxane is anion-polymerized with a lithium trialkylsilanolate as an initiator, which is then reacted with a chlorosilane which has a (meth)acryl group, such as 3-(2-methacryloyloxy ethoxy)propyl dimethyl chlorosilane to prepare the silicone represented by the following formula (b).

In the afore-mentioned method, a silicone chain which has a high polymerization degree of the cyclic siloxane is formed. When the silicone is mixed with a hydrophilic monomer, such as 2-hydroxyethyl methacrylate, turbidity occurs sometimes. Additionally, a ratio of the end-blocking of the silicone chain by the chlorosilane is not high.
Another method is also known from Patent Literature 4, where a silicone represented by the following formula (c) is prepared by esterifying a (meth)acrylic acid or transesterifying a (meth)acrylate with an organopolysiloxane having a hydroxyl group on its one end,
                wherein r is an integer of 3 or larger.        
In the above-described method, the esterification ratio is insufficient, the end blocking ratio is low, and the distribution of the degree of polymerization of the silicone is broad. Patent Literature 4 refers also to a preparation method where an unsaturated compound having a (meth)acryl group is hydrosililated with an organopolysiloxane having an SiH group on its one end. However, in this method, the hydrosililation of the (meth)acrylic moiety of the unsaturated compound having a (meth)acryl group also takes place and, therefore, the desired compound cannot be obtained in a high purity.    [Patent Literature 1]Japanese Patent Application Laid-Open No. 2007-186709    [Patent Literature 2]Japanese Patent Application Laid-Open No. 2007-1918    [Patent Literature 3]Japanese Patent Application Laid-Open No. Sho-59-78236    [Patent Literature 4]Japanese Patent Application Laid-Open No. 2001-55446